Problem: Solve for $t$ : $-12 = 9 + t$
Answer: Subtract $9$ from both sides: $ \begin{eqnarray} \\ \hphantom{-12 {- 9}} &\hphantom{=}& \hphantom{t} \\ -12 &=& 9 + t \\ \\ {-9} && {-9} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -12 {- 9} &=& t \\ \end{eqnarray} $ Simplify. $ t = -21$